a) Field of the Invention
The invention is directed to a method and an arrangement for the electronic recording of a moving object, particularly for recording a finger that is rolled on a support surface. The invention is applied chiefly for preparing electronic fingerprints but can also be advantageously used in object tracking for intelligent control of a sampling window that is variable with respect to position and/or size.
b) Description of the Related Art
The basis for the use of fingerprinting in criminology lies in the uniqueness of dermal ridge images which are not inheritable and which are inalterable from the fourth month of embryonic life until their dissolution following death. The master image may be the finger itself, a fingerprint made upon paper by ink, or a trace photogram. The latter two techniques were and are used primarily by police. It is possible to automate the identification and verification of fingerprints by means of pattern recognition techniques which are capable of extracting the characterizing features of a fingerprint.
In recent years, the techniques mentioned above have increasingly been integrated into electronic systems which permit direct recording of the finger. This shortens recording and evaluating times so that the quality of the images can be improved at the same time. Therefore, when a finger is not correctly recorded, it is possible to repeat the process of recording this finger immediately.
Electronic image recording of fingerprints is usually carried out with matrix sensors or line sensors based on CCD or CMOS technology. In so doing, the fingerprint, as master image, is converted through special optics and sensors into an electronic image and is subsequently digitized to form a screen image with a fixed spatial and gray value resolution.
However, capacitive, thermal, ultrasound-based or pressure-sensitive sensors can also be used for electronic recording of fingerprints.
Besides flat printing of the finger, images of rolled fingers, above all, are also made by police. The procedure for producing rolled fingerprints with ink and paper is sufficiently simple: ink is applied to the finger and the finger is then rolled on paper. In so doing, rotation and deformations or smearing at the ends of the finger are accepted and permissible.
This procedure is somewhat more involved in electronic systems. In this case, depending on the manner in which the finger is electronically acquired (with line or matrix), a total image is put together from many individual images. In this connection, there is the problem that, in contrast to rolling with ink on paper, there is not a complete recording but rather a plurality of discrete-time samples which involves a loss of information. Apart from the deformation of the finger and the resulting changes at two different points in time, the reasons for the loss of information reside particularly in that the duration of the scan of the finger by the image recording unit is too short. This means that in order to ensure that a finger is correctly recorded, the image recording unit—depending on the method of composition that is employed—must have a minimum image rate so that interfering processes such as twisting or slipping during the process of rolling the finger can be detected.
Methods for arriving at a total image with minimal errors based on the inevitable individual images in which the generation of the image of a rolled finger is always preceded by a successively recorded series of individual electronic images have been described many times. The total image is put together from these individual images by means of many different methods in which the individual images are divided into slices or strips.
In U.S. Pat. No. 4,933,976, U.S. Pat. No. 5,230,025, U.S. Pat. No. 6,483,932, WO 97/41528 and DE 198 51 544 C1 (WO 00/28470), a strip is taken from each of the individual images and the total image is assembled from these strips. The patents mentioned above are distinguished from one another by the different algorithms by which the strips are determined and assembled in order, as far as possible, to join only those parts of the individual images that correspond to the actual contact surface of the finger.
In all of the references mentioned above, the individual images coming from the image recording unit have a fixed size and are based on a fixed time regime (given by a clock generator or an event trigger). Accordingly, the rolling speed (and possibly a change in the rolling speed) and the size of the finger contact surface (and real changes in the latter during the roll processes) cannot be taken into account in the readout regime of the image recording unit.
In order to make it possible to correctly calculate the resulting image, two adjacent fingerprint strips must have an intersect that is sufficiently large for the method in question. This can be achieved only when a fast, timed sampling is carried out while the finger is rolled. Currently, this is realized by means of sensors with a high image readout speed (image rate) of more than 25 images/second +. However, technical limitations are encountered when the spatial resolution is increased (e.g., from 500 dpi to 1000 dpi). Either the image rate is decreased (e.g., image rate reduced to ¼ when changing from 500 dpi to 1000 dpi) or the price of the sensor, including its electronics, is prohibitively high. + [(I/S), usually known as frames per second (fps)]